1. Field of the Invention
The invention relates to a switching arrangement, or control circuit, for operating a high-pressure sodium lamp which radiates white light in stable operating conditions, which switching arrangement is provided with switching means for switching current through the lamp by means of a drive signal generated in a drive circuit and derived from a comparison between a reference value C and a drive signal having the form V+.beta.I, where
V is the lamp voltage, PA1 I is the lamp current, and PA1 .beta. is a constant.
2. Description of the Prior Art
A switching arrangement of the kind described in the opening paragraph is known from the European Patent Application EP-A-0228123. An important characteristic of the known switching arrangement is that the lamp voltage is kept constant by fair approximation, so that the colour temperature T.sub.c of the light radiated by the lamp remains within acceptable limits during a longer period. It is important to limit the change of the colour temperature T.sub.c in view of the characteristic that the lamp radiates "white light". As a rule, it is true for these lamps that the colour temperature T.sub.c &gt;2250 K. The area in the colour triangle within which the light of a high-pressure sodium lamp is called "white" is limited by straight lines through the points having coordinates (x, y): (0.400; 0.430), (0.510; 0.430), (0.485; 0.390) and (0.400; 0.360). The colour temperature T.sub.c in that case lies between approximately 2300 K. and 4000 K. According to more stringent requirements, based on a better acceptation of the light by observers, the light is called " white" if it lies in an area of the colour triangle bounded by the lines x=0.468, x=0.490, y=0.408 and y=0.425. The colour temperature then lies between approximately 2300 K. and approximately 2700 K. Lamps of the type described may be used to replace incandescent lamps.
It has been found, however, that the reasonable maintainance at a constant level of the lamp voltage by means of the known switching arrangement does not prevent the colour temperature T.sub.c showing a drift during lamp life and generally falling to such a level that the colour point of the light radiated by the lamp will move outside the area indicated as the area of "white light".
Based on the colour of the light radiated by the lamp then, the lamp can be regarded as having reached the end of its "white" life. The lamp has by no means reached the end of its electrical life then, however.